1. Field of the Invention
The present invention relates generally to the separation of wood pulp fibers into various fractions such as long and short fibers for efficiencies in the paper making process, especially concerning paper mill effluents containing pulp slurry recyclable through separation of large radius fibers and small radius fibers into a concentrated paper fiber stock of the relatively larger fibers for paper making and a clean stream of the relatively smaller fibers from a stream of dilute fiber suspension. More particularly the invention relates to an acoustic cell, an acoustic fractionator and methods of separating dilute suspensions of fibers for fiber fractionation with acoustic separation using acoustic wave fields. Such acoustic wave fields induce deflections of the fibers penetrating an ultrasonic field imposing agglomeration and reorientation on the fibers of fiber suspensions to separate the fibers into two or more fractions according to the relative sizes of the fibers.
2. Description of the Related Art
Separation of virgin or reclaimed wood pulp fibers into two or more fractions which are relatively enriched in longer or shorter fibers is an important step of the paper-making process, because it allows for the efficient use of fiber properties. Fiber fractionation allows an optimized use of raw materials, increases production versatility, and contributes to waste and energy consumption reduction. Typical examples are: optimization of multi-layered products by placing fractions where they are most needed in the sheet; energy savings by restricting pulp refining to the long-fiber fraction; separation of valuable fraction from waste.
Various technologies have been devised during the past forty years to fractionate wood pulp fibers. See, e.g., P. Seifert and K. L. Long, "Fiber fractionation--Methods and Applications," Tappi J., Vol. 57(10) pp. 69-72 (1974); and T. F. Skaar, "Fractionation: Equipment, Applications and Process Control," Proc. of the Pulping Conf., Tappi Press, pp. 211-216 (1984). Pressure screen systems, which fractionate fibers based on fiber length, are generally perceived as the most successful technology on a commercial stand-point. See, H. Ibrahim, "Fiber Fractionation--An appropriate Technology for Upgrading Recycled Fibers and Saving Energy," Proc. Eucepa Symp. "Recycling of Fibres and Fillers in Pulp and Paper Industry" Ljubljana pp. 539-569 (Oct. 23-27, 1989).
In pressure screen systems, pulp slurries circulate between a stationary cylinder-shape screen and an external rotor. Pressure conditions between the screen and the rotor are such that the short fibers pass through the screen; long fibers are retained on the screen. The separation efficiency depends upon pulp furnish, use of perforated or slotted screens, pulp consistency, and input and output flow rates. The technology has high throughput. However, pressure screens have limited on-line adaptability to variable pulp furnishes, and they also have limited on-line adaptability for variable product requirements. Also, separation efficiency is not always satisfactory and multi-stage systems are used to meet separation goals. These are drawbacks when one considers the ever increasing use of reclaimed fibers from mixed grades. Moreover, pressure screen systems cannot fulfill fractionation requirements based on fiber radius, wall thickness, or coarseness, especially the separation of springwood and summerwood fibers or the separation of softwood and hardwood fibers, or in the separation of shives from the fibers.
It would be desirable to employ a versatile mechanism facilitating on-line adaptability to variable pulp furnishes and variable product requirements such as fiber fractionation with acoustic separation using ultrasonic wave fields, but very little knowledge has been available on the interaction of a sound field with fibers and more generally with prolate spheroids and cylindrical particles. Awatani was the first to calculate the acoustic force on a prolate spheroid, J. Awatani, "On the Acoustic Radiation Pressure on a Prolate Spheroid," Memo. Inst. Sci. Ind. Res., Osaka U., Vol. 10, pp. 51-65 (1953), and a rigid circular cylinder, J. Awantani, "Study on Acoustic Radiation Pressure (IV), Radiation Pressure on a Cylinder," Memo.
Inst. Sci. Ind. Res., Osaka U., Vol. 12, pp. 95-102 (1955), in plane traveling and standing wave fields. He found that the force on a prolate spheroid, whose axis of symmetry is perpendicular to the sound field direction (stable orientation), is larger than that on a disc or a sphere which has the same projective area. More recently, Zhuck and Wu et al. reported independent derivations of the acoustic force on a rigid cylinder at stable orientation, in plane traveling and standing wave fields, respectively. See, A. P. Zhuck, "Radiation Force Acting on a Cylinder Particle in a Sound Field," Sov. Appl. Mech., Vol. 22, pp. 689-693 (1987); and J. Wu G. Du, S. S. Work and D. M. Warshaw, "Acoustic Radiation Pressure on a Rigid Cylinder: An Analytical Theory and Experiments," J. Acoust. Soc. Am., Vol. 87, pp. 581-586 (1990).